1. Basic in threads
Threading terms and definitions
1. Root/bottom
The bottom surface joining the two adjacent flanks of
the thread
2. Flank/side
The side of a thread surface connecting the crest and
the root
3. Crest/top
The top surface joining the two sides, or flanks.
P = Pitch, mm or threads per inch (t.p.i.)
= The profile angle
= The helix angle of the thread
d / D = The major diameter, external/internal
d1 / D1 = The minor diameter, external/internal
d2 / D2 = The pitch diameter, external/internal
Pitch diameter, d2 / D2
This is the effective diameter of the screw
thread; approximately half way between the
major and minor diameters.
Helix angle
The helix angle ()is the geometrical shape
of the screw thread, it is based on the pitch
diameter of the thread (d2, D2), and the
pitch (P) – the distance from one point on a
thread profile to the corresponding point on
the next.
This measurement can be represented by
a triangle being unwound from the component.
4
The same pitch on different diameters gives a different helix angle,
see example above.
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